a real simple way of looking at it from a gambling point of view, without even doing any actual research or analysis on stocks (the stuff that matters)
if you total the net loss/profit for gambling on pokies year after year for decades, its always a massive loss at the gamblers expense and it will forever be that way.
Stock markets rise in general over the decades unless your country is failing. So in theory the odds are in your favour rather than not.
The less stocks the less probability of following the trend. Just as only investing in one industry gives you less chance.